1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile transceiver or similar photographic image forming apparatus in which an image is formed on a photoconductive element. More particularly, the invention is concerned with a contact type image transferring device including, for example, a transfer roller or a transfer belt for transferring a toner image from the photoconductive element to a sheet of paper which is passed through a nip between the photoconductive element and the image transferring device. The present invention further relates to a method and apparatus for electrically cleaning the transferring device.
2. Discussion of the Background
It is a common practice for an image forming apparatus of the type described above to use a contact type image transferring device. The contact type image transferring device such as a transfer roller has applied thereto an electrical field opposite in polarity to the polarity of a toner image on a photoconductive element. The image transferring device transfers the toner image from the photoconductive element to a sheet passed through a nip between the photoconductive element and the transfer device. Since the contact type transfer device is in direct contact with the photoconductive element when the sheet is not at the nip, the toner image on the surface of the photoconductive element transfers to the surface of the transfer device. Subsequently, the toner image on the transfer device is transferred to the back side of the sheet.
Japanese Laid-Open Patent No. 3-69978 discloses a cleaning device for a transfer roller in which toner on the surface of the roller is transferred to the photoconductive element by applying cleaning bias voltage to the transfer roller when the transfer roller is in direct contact with the photoconductive element. Namely, the cleaning bias voltage is applied during a pre-image forming time period (i.e., from the time the photoconductive element starts its rotation until the leading edge of an image area on the photoconductive element reaches the nip), an inter-image forming time period (i.e., between successive copying operations), and a post-image forming time period (i.e., after the last image area on the photoconductive element passes through the nip). Since there is not only regularly charged toner having a positive polarity but also oppositely charged toner having a negative polarity, for cleaning both types of toner, this publication discloses that the polarity of a cleaning bias voltage is switched over between the positive polarity and the negative polarity.
However, in Japanese Laid-Open Patent No. 3-69978, since the cleaning operation is executed every time at the pre-image forming period, the inter-image time, and the post-image forming time, it is always necessary to have a waiting period for the bias cleaning operation.
Further, if a large quantity of toner is adhered to the surface of the transfer roller, the cleaning ability becomes poor since the cleaning time period at the inter-image time is very short and therefore, some toner remains on the transfer roller.
The condition of a large quantity of toner adhering to the surface of the transfer roller occurs when the sheet of paper is jammed. If the sheet of paper is jammed, toner on the surface of the photoconductive element is directly transferred to the transfer roller because the sheet of paper is not fed to the nip and consequently the transfer roller is in direct contact with the photoconductive element. The toner on the transfer roller is then transferred to the back side of the sheet of paper after the jammed sheet of paper is removed and the next image forming operation is started.
Japanese Laid-Open Patent No. 5-341671 discloses a cleaning device for a transfer roller in which after the paper feed jam is corrected, a negative polarity cleaning bias voltage which is the same polarity as the regularly charged toner is applied to the transfer roller for two seconds. Then the cleaning bias voltage is switched over from the negative polarity to the positive polarity and the positive polarity cleaning bias voltage is applied to the transfer roller for two seconds. A timing diagram of the voltage applied to the transfer roller in order to clean the transfer roller is illustrated in FIG. 8. In FIG. 8, time A is the start of the cleaning process, time B is the time at which the voltage is switched in polarity and is two seconds after the cleaning operation starts, and time C is the end of the cleaning operation and is four seconds after the cleaning operation starts.
There is a general trend of reducing the time necessary to perform various operations in photoconductive devices. Therefore if this trend were followed with the teachings related to the cleaning operation performed in JP 5-341671, it would appear to be desirable to reduce the time of cleaning (i.e., reduce the time duration during which the clean bias voltages are applied to the transfer roller).
The present inventors have noticed that when the voltage is changed from the positive polarity to the negative polarity, positively charged toner which has been transferred from the transfer roller to the photoconductive drum may be improperly transferred back to the transfer roller. The inventors have also noticed that when the cleaning voltage is changed from the positive polarity to the negative polarity during the cleaning operation, there may be a voltage spike or over-shoot of the desired negative polarity voltage, as illustrated in FIG. 8 at time B. This voltage spike or overshoot causes a very strong attraction of toner on the photoconductive drum which is contacting or is near the transfer roller and results in an undesirable transfer of toner back to the transfer roller. This toner which is transferred back to the transfer roller is then undesirably transferred to the back of the next sheet of paper passing between the photoconductive drum and the transfer roller.